1. Field of the Invention
This invention relates to gallium arsenide (GaAs) growth crystals formed on silicon (Si) substrate which are necessary for producing such semiconductors as are employed, for example, in solar cells, microwave-communication devices and the like and to a method of growing such crystals.
2. Description of the Prior Art
As a semiconductor employed in the above mentioned devices, a conventional crystal is known which consists of a GaAs layer grown on a GaAs substrate [for example, Journal of Crystal Growth, 55 (1981) 213-222, North-Holland Publishing Company]. Such a semiconductor, however, is not practical since it costs too much to produce.
The inventors tried to make use of a Si substrate. When a GaAs single crystal is grown on a Si substrate, according to the inventors' experience, an intermediate layer of germanium (Ge) must first be formed on the Si substrate, for example, by evaporation as a kind of moderator material to absorb the mismatching effects originating from the differences between the lattice constants and between the coefficients of thermal expansion of Si and GaAs. The GaAs growth crystal on the Si substrate is then obtained by growing a GaAs layer on the intermediate layer.
The provision of the GaAs layer onto Si substrate by this method, however, requires such special technical steps other than the metalorganic chemical vapor deposition (MOCVD) process, as the ion cluster beam or vacuum evaporation technique. These additional steps unduly increase the cost of production of the growth crystal, and the performance of produced growth crystals widely diverges, owing to the variable crystalline properties governed notably by the structural condition of the formed Ge layer. It is a further shortcoming that the photoluminescence (PL) intensity of the GaAs layer provided on the Ge layer is smaller by one order of magnitude than that of the uppermost layer of a GaAs crystal grown, for example, on a GaAs substrate.